Polystyrene is well known as a leading plastic, produced on a large scale, usually in continuous polymerization plants. Modified forms of polystyrene, such as the use of butadiene based rubber to increase impact resistance of polystyrene, are also well known.
The largest area of use for polystyrene based plastics is in packaging applications, e.g., disposable tumblers, television cabinets, meat and food trays, and egg cartons. Other uses include computer cabinets and telephone equipment where a combination of high impact strength and attractive appearance, including gloss, are desirable.
Examples of references to production of high impact polystyrene polyblends, commonly called "HIPS", comprising polystyrene having a rubber phase dispersed therein, as crosslinked rubber particles, include U.S. Pat. No. 3,903,202 to Carter et al.
There are numerous, more recent examples of HIPS product containing bimodal distribution of rubber particles. U.S. Pat. No. 4,146,589 to Dupre discloses a HIPS product having rubber particles of about 0.5 to 1.0 microns and of about 2 to 3 microns. In accordance with U.S. Pat. No. 4,146,589, a preferred rubber is formed by the polymerization of 1,3-butadiene wherein the butadiene has a cis-isomer content of about 30-99.5% and a trans-isomer content of about 70-2%, and generally contain at least about 85% of polybutadiene formed by 1,4 addition with no more than about 15% by 1,2 addition.
U.S. Pat. No. 4,493,922 to Echte et al. discloses a HIPS product having bimodal rubber particle size distribution of 0.2 to 0.6 microns and particles of 2 to 8 microns. According to Echte et al., the rubber or elastomeric phase can be composed of polybutadiene or of block copolymers of butadiene and styrene. The smaller rubber particles have a so-called capsule morphology and the larger particles have a cell morphology.
Bragaw, in an article titled "The Theory of Rubber Toughening of Brittle Polymers", published in Multicomponent Polymer Systems, Advances in Chemistry Series 99, ACS, 1971, pp. 86-106, discusses the use of different particle sizes for a dispersed rubber phase to raise the impact strength of polystyrene.
U.S. Pat. No. 5,039,714 to Kasahara et al. discloses an impact resistant rubber modified polystyrene composition of high gloss having bimodal rubber particle size distribution of 0.1 to 0.6 microns with a single occlusion (capsule) structure and a larger particle component of 0.7 to 1.9 microns with a cellular particle structure.
U.S. Pat. No. 5,294,656 to Okamoto et al. discloses a rubber modified polystyrene based composition which is improved, compared to ABS, in balance between impact strength and appearance of shaped articles such as gloss and image clarity. According to U.S. Pat. No. 5,294,656, the polystyrene based composition has a bimodal rubber particle size distribution of (a) 0.1 to 0.4 microns having a core/shell (capsule) structure and (b) 0.8 to 2.0 microns having a cell structure, and wherein the resin contains methyl phenyl silicone oil. The rubber elastomer in U.S. Pat. No. 5,294,656 may be a polybutadiene or a styrene-butadiene copolymer.